i would guess that the new PSU isnt just weaker,
but: it also has higher ripple noise...
maybe a low pass helps?
or a 1mF capacitor across the new PSU?
+9V ----- LLLLL -------- Vout
|
===== C
|
GND----- LLLLL -------- GND'
2nd theory:
maybe the new PSU is not isolated from mains ground? XD